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Message-Id: <1489160052-20293-1-git-send-email-dwindsor@gmail.com>
Date: Fri, 10 Mar 2017 10:34:12 -0500
From: David Windsor <dwindsor@...il.com>
To: peterz@...radead.org,
mingo@...nel.org,
elena.reshetova@...el.com,
dwindsor@...il.com
Cc: linux-kernel@...r.kernel.org,
kernel-hardening@...ts.openwall.com
Subject: [PATCH v3] refcount: add refcount_t API kernel-doc comments
v3: rebase against latest Linus tree; re-add include/linux/refcount.h
missing from v2 series
Signed-off-by: David Windsor <dwindsor@...il.com>
---
include/linux/refcount.h | 19 ++++++++
lib/refcount.c | 124 ++++++++++++++++++++++++++++++++++++++++++-----
2 files changed, 130 insertions(+), 13 deletions(-)
diff --git a/include/linux/refcount.h b/include/linux/refcount.h
index 0023fee..b34aa64 100644
--- a/include/linux/refcount.h
+++ b/include/linux/refcount.h
@@ -6,17 +6,36 @@
#include <linux/spinlock.h>
#include <linux/kernel.h>
+/**
+ * refcount_t - variant of atomic_t specialized for reference counts
+ * @refs: atomic_t counter field
+ *
+ * The counter saturates at UINT_MAX and will not move once
+ * there. This avoids wrapping the counter and causing 'spurious'
+ * use-after-free bugs.
+ */
typedef struct refcount_struct {
atomic_t refs;
} refcount_t;
#define REFCOUNT_INIT(n) { .refs = ATOMIC_INIT(n), }
+/**
+ * refcount_set - set a refcount's value
+ * @r: the refcount
+ * @n: value to which the refcount will be set
+ */
static inline void refcount_set(refcount_t *r, unsigned int n)
{
atomic_set(&r->refs, n);
}
+/**
+ * refcount_read - get a refcount's value
+ * @r: the refcount
+ *
+ * Return: the refcount's value
+ */
static inline unsigned int refcount_read(const refcount_t *r)
{
return atomic_read(&r->refs);
diff --git a/lib/refcount.c b/lib/refcount.c
index aa09ad3..8d5d680 100644
--- a/lib/refcount.c
+++ b/lib/refcount.c
@@ -37,6 +37,24 @@
#include <linux/refcount.h>
#include <linux/bug.h>
+/**
+ * refcount_add_not_zero - add a value to a refcount unless it is 0
+ * @i: the value to add to the refcount
+ * @r: the refcount
+ *
+ * Will saturate at UINT_MAX and WARN.
+ *
+ * Provides no memory ordering, it is assumed the caller has guaranteed the
+ * object memory to be stable (RCU, etc.). It does provide a control dependency
+ * and thereby orders future stores. See the comment on top.
+ *
+ * Use of this function is not recommended for the normal reference counting
+ * use case in which references are taken and released one at a time. In these
+ * cases, refcount_inc(), or one of its variants, should instead be used to
+ * increment a reference count.
+ *
+ * Return: false if the passed refcount is 0, true otherwise
+ */
bool refcount_add_not_zero(unsigned int i, refcount_t *r)
{
unsigned int old, new, val = atomic_read(&r->refs);
@@ -64,19 +82,40 @@ bool refcount_add_not_zero(unsigned int i, refcount_t *r)
}
EXPORT_SYMBOL_GPL(refcount_add_not_zero);
+/**
+ * refcount_add - add a value to a refcount
+ * @i: the value to add to the refcount
+ * @r: the refcount
+ *
+ * Similar to atomic_add(), but will saturate at UINT_MAX and WARN.
+ *
+ * Provides no memory ordering, it is assumed the caller has guaranteed the
+ * object memory to be stable (RCU, etc.). It does provide a control dependency
+ * and thereby orders future stores. See the comment on top.
+ *
+ * Use of this function is not recommended for the normal reference counting
+ * use case in which references are taken and released one at a time. In these
+ * cases, refcount_inc(), or one of its variants, should instead be used to
+ * increment a reference count.
+ */
void refcount_add(unsigned int i, refcount_t *r)
{
WARN_ONCE(!refcount_add_not_zero(i, r), "refcount_t: addition on 0; use-after-free.\n");
}
EXPORT_SYMBOL_GPL(refcount_add);
-/*
- * Similar to atomic_inc_not_zero(), will saturate at UINT_MAX and WARN.
+/**
+ * refcount_inc_not_zero - increment a refcount unless it is 0
+ * @r: the refcount to increment
+ *
+ * Similar to atomic_inc_not_zero(), but will saturate at UINT_MAX and WARN.
*
* Provides no memory ordering, it is assumed the caller has guaranteed the
* object memory to be stable (RCU, etc.). It does provide a control dependency
* and thereby orders future stores. See the comment on top.
- */
+ *
+ * Return: true if the increment was successful, false otherwise
+ */
bool refcount_inc_not_zero(refcount_t *r)
{
unsigned int old, new, val = atomic_read(&r->refs);
@@ -103,11 +142,17 @@ bool refcount_inc_not_zero(refcount_t *r)
}
EXPORT_SYMBOL_GPL(refcount_inc_not_zero);
-/*
- * Similar to atomic_inc(), will saturate at UINT_MAX and WARN.
+/**
+ * refcount_inc - increment a refcount
+ * @r: the refcount to increment
+ *
+ * Similar to atomic_inc(), but will saturate at UINT_MAX and WARN.
*
* Provides no memory ordering, it is assumed the caller already has a
- * reference on the object, will WARN when this is not so.
+ * reference on the object.
+ *
+ * Will WARN if the refcount is 0, as this represents a possible use-after-free
+ * condition.
*/
void refcount_inc(refcount_t *r)
{
@@ -115,6 +160,26 @@ void refcount_inc(refcount_t *r)
}
EXPORT_SYMBOL_GPL(refcount_inc);
+/**
+ * refcount_sub_and_test - subtract from a refcount and test if it is 0
+ * @i: amount to subtract from the refcount
+ * @r: the refcount
+ *
+ * Similar to atomic_dec_and_test(), but it will WARN, return false and
+ * ultimately leak on underflow and will fail to decrement when saturated
+ * at UINT_MAX.
+ *
+ * Provides release memory ordering, such that prior loads and stores are done
+ * before, and provides a control dependency such that free() must come after.
+ * See the comment on top.
+ *
+ * Use of this function is not recommended for the normal reference counting
+ * use case in which references are taken and released one at a time. In these
+ * cases, refcount_dec(), or one of its variants, should instead be used to
+ * decrement a reference count.
+ *
+ * Return: true if the resulting refcount is 0, false otherwise
+ */
bool refcount_sub_and_test(unsigned int i, refcount_t *r)
{
unsigned int old, new, val = atomic_read(&r->refs);
@@ -140,13 +205,18 @@ bool refcount_sub_and_test(unsigned int i, refcount_t *r)
}
EXPORT_SYMBOL_GPL(refcount_sub_and_test);
-/*
+/**
+ * refcount_dec_and_test - decrement a refcount and test if it is 0
+ * @r: the refcount
+ *
* Similar to atomic_dec_and_test(), it will WARN on underflow and fail to
* decrement when saturated at UINT_MAX.
*
* Provides release memory ordering, such that prior loads and stores are done
* before, and provides a control dependency such that free() must come after.
* See the comment on top.
+ *
+ * Return: true if the resulting refcount is 0, false otherwise
*/
bool refcount_dec_and_test(refcount_t *r)
{
@@ -154,21 +224,26 @@ bool refcount_dec_and_test(refcount_t *r)
}
EXPORT_SYMBOL_GPL(refcount_dec_and_test);
-/*
+/**
+ * refcount_dec - decrement a refcount
+ * @r: the refcount
+ *
* Similar to atomic_dec(), it will WARN on underflow and fail to decrement
* when saturated at UINT_MAX.
*
* Provides release memory ordering, such that prior loads and stores are done
* before.
*/
-
void refcount_dec(refcount_t *r)
{
WARN_ONCE(refcount_dec_and_test(r), "refcount_t: decrement hit 0; leaking memory.\n");
}
EXPORT_SYMBOL_GPL(refcount_dec);
-/*
+/**
+ * refcount_dec_if_one - decrement a refcount if it is 1
+ * @r: the refcount
+ *
* No atomic_t counterpart, it attempts a 1 -> 0 transition and returns the
* success thereof.
*
@@ -178,6 +253,8 @@ EXPORT_SYMBOL_GPL(refcount_dec);
* It can be used like a try-delete operator; this explicit case is provided
* and not cmpxchg in generic, because that would allow implementing unsafe
* operations.
+ *
+ * Return: true if the resulting refcount is 0, false otherwise
*/
bool refcount_dec_if_one(refcount_t *r)
{
@@ -185,11 +262,16 @@ bool refcount_dec_if_one(refcount_t *r)
}
EXPORT_SYMBOL_GPL(refcount_dec_if_one);
-/*
+/**
+ * refcount_dec_not_one - decrement a refcount if it is not 1
+ * @r: the refcount
+ *
* No atomic_t counterpart, it decrements unless the value is 1, in which case
* it will return false.
*
* Was often done like: atomic_add_unless(&var, -1, 1)
+ *
+ * Return: true if the decrement operation was successful, false otherwise
*/
bool refcount_dec_not_one(refcount_t *r)
{
@@ -219,13 +301,21 @@ bool refcount_dec_not_one(refcount_t *r)
}
EXPORT_SYMBOL_GPL(refcount_dec_not_one);
-/*
+/**
+ * refcount_dec_and_mutex_lock - return holding mutex if able to decrement
+ * refcount to 0
+ * @r: the refcount
+ * @lock: the mutex to be locked
+ *
* Similar to atomic_dec_and_mutex_lock(), it will WARN on underflow and fail
* to decrement when saturated at UINT_MAX.
*
* Provides release memory ordering, such that prior loads and stores are done
* before, and provides a control dependency such that free() must come after.
* See the comment on top.
+ *
+ * Return: true and hold mutex if able to decrement refcount to 0, false
+ * otherwise
*/
bool refcount_dec_and_mutex_lock(refcount_t *r, struct mutex *lock)
{
@@ -242,13 +332,21 @@ bool refcount_dec_and_mutex_lock(refcount_t *r, struct mutex *lock)
}
EXPORT_SYMBOL_GPL(refcount_dec_and_mutex_lock);
-/*
+/**
+ * refcount_dec_and_lock - return holding spinlock if able to decrement
+ * refcount to 0
+ * @r: the refcount
+ * @lock: the spinlock to be locked
+ *
* Similar to atomic_dec_and_lock(), it will WARN on underflow and fail to
* decrement when saturated at UINT_MAX.
*
* Provides release memory ordering, such that prior loads and stores are done
* before, and provides a control dependency such that free() must come after.
* See the comment on top.
+ *
+ * Return: true and hold spinlock if able to decrement refcount to 0, false
+ * otherwise
*/
bool refcount_dec_and_lock(refcount_t *r, spinlock_t *lock)
{
--
2.7.4
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